What material should go in the red outline? (Details and picture below)

Hello there. I am going to be building an emerald factory on my second residence. It will be three farms that get you materials that you can trade with villagers to get emeralds. It will be at res # 1134, smp1. I currently do not have any expected finiching date, but hopefully it will be before my birthday. (April 16) I also need to conduct a survey regarding my emerald factory. Take a look at this picture here:See the areas outlined in red? I need to fill those with some block, but I simply don't know what. That is what the poll above is for, what two blocks to put in those areas. When the poll is over, I will use the top two voted blocks and use them in those areas. if two are tied for second, I will choose the next block out of those two that someone says here. I will pick the 2 winning blocks on new year's eve. (Iknow I did a bad job with the picture, I spent about 30 seconds in paint. )So that is all, goodbye, and don't forget to vote!

I will be ending early! The walls will be made of obsidian and either end stone or smooth sandstone. For a tiebreaker, the next person to say "Sandstone!" or "Endstone!" Will decide the second material. I will finish the walls immediatly after the second item is decided.

Sandstone goes beautifully with green wool. If you do use nether brick, I would use it only barely,and with the sandstone, otherwise it just looks weird. Endstone, in my opinion, is ugly, and only looks good when used as a path or road. But that's just my opinion. Having entirely glass walls will make it look like a generic EMC mall, and while obsidian might look good, you really don't want to have to tear all the obsidian down if it turns out it doesn't look good. xP

Sandstone goes beautifully with green wool. If you do use nether brick, I would use it only barely,and with the sandstone, otherwise it just looks weird. Endstone, in my opinion, is ugly, and only looks good when used as a path or road. But that's just my opinion. Having entirely glass walls will make it look like a generic EMC mall, and while obsidian might look good, you really don't want to have to tear all the obsidian down if it turns out it doesn't look good. xP